The present invention relates to a resource recovery method. More particularly, the present invention relates to a resource recovery method using a multi-stage submerged membrane distillation water treatment apparatus including a series of raw water tanks arranged in multiple stages, each tank storing raw water and provided with a membrane distillation (MD) module and a heat exchanger submerged in the raw water, the apparatus being configured such that vapor discharged from a membrane distillation module at a certain stage is used for heat exchange by a heat exchanger at the following stage, whereby the apparatus dramatically reduces consumption of energy required for heating of the raw water and recovers resources contained in the raw water.
According to a 2009 World Economic Forum Water Resource Initiative report, the world water demand increased three-fold from 1950 to 1990, which was an increase rate greatly higher than an increase rate of the world population. It was also noted that water demand is expected to increase two-fold within the next 35 years. In addition, the current active production of alternative energy resources such as hydrogen gas and bioethanol, will further increase water demand.
To address water shortages, recycling and reuse of sewage or waste water have recently been actively studied. However, in fact, a water reuse rate is currently actually very low, or usage of recycled water is very limited due to concerns about the quality of the recycle water. For example, recycled water is largely used as cleaning water in sewage and waste water treatment facilities, or as cooling water or diluting water.
To solve this problem, various technologies are now being used in water treatment or reuse sites. For example, membrane distillation (MD), which is a water treatment technology of separating water in the form of pure vapor from raw water using a hydrophobic porous membrane, is used.
In a membrane distillation process, raw water comes into contact with the surface of a membrane but cannot permeate into the pores of the membrane due to high surface tension attributable to the highly hydrophobic surface of the membrane, and only vapor passes through the pores of the membrane and is collected as fresh water.
The cause of mass transfer in a membrane distillation process is due to a temperature difference between high-temperature raw water and low-temperature filtrate divided by a separation membrane. A vapor pressure difference triggered by the temperature difference is the driving force of causing the vapor, changed from liquid phase water, to move from the raw water side to the filtrate side.
Membrane distillation methods are classified into four categories according to technologies applied to the filtrate side to generate a vapor pressure gradient serving as the driving force: direct contact membrane distillation (DCMD); air gap membrane distillation (AGMD); sweep gas membrane distillation (SGMD); and vacuum membrane distillation (VMD).
Membrane distillation methods treat raw water based on phase change. The methods have many advantages: approximately 100% of removal rate of nonvolatile contaminants; lower operation pressures compared to reverse osmosis; and simple pretreatment equipment and facilities due to its high resistance to membrane contamination.
However, despite of these advantages, the methods also have a disadvantage that they require high energy consumption to heat raw water to a predetermined temperature (typically 60 to 80° C.) to generate a vapor pressure difference serving as the driving force of mass transfer. In addition, energy is further consumed to condense the vapor having passed through a membrane distillation module.
In addition, conventional membrane distillation methods have difficulty in removing contaminants from sewage or waste water in which highly concentrated salts (i.e. valuable resources) are contained, and are thus poor at producing quality fresh water. Furthermore, conventional membrane distillation methods cannot recover or recycle valuable resources contained in high concentration, and thus the valuable resources are chemically treated and collected as sludge. As a result, a large amount of chemicals is necessarily used and correspondingly a large amount of sludge is generated, which results in an increase in sludge treatment costs.
For these reasons, development of a technology of improving energy efficiency and efficiently recovering valuable resources contained in raw water in a membrane distillation process is required.
The foregoing is intended merely to aid in the understanding of the background of the present invention, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art.